The present invention relates to chemically durable porous glass and a process for the manufacture thereof. More particularly, the invention relates to novel chemically durable porous glass useful for various separating media or membranes or as filling material for use in gel permeation chromatography, and also to a process for the manufacture thereof by a phase separation method.
With reference to U.S. Pat. Nos. 2,106,774 and 3,843,341 for example, thereby it is known to produce porous glass by heat treating at a temperature of 500.degree. to 650.degree. C. borosilicate glass mainly comprising silicic acid, boric acid, alumina and alkali substances to cause to occur a phase separation between a phase mainly comprising SiO.sub.2 and a phase mainly comprising B.sub.2 O.sub.3, and then by selectively eluting the latter phase with a mineral acid.
However, the porous glasses produced by such method have a skeleton mainly comprising SiO.sub.2 and are therefore poor in the waterproof property and the resistivity to alkali, and cannot be suitably employed for use in contact with water or alkali.
Also, during the phase separation, some amount of SiO.sub.2 tends to mingle into the B.sub.2 O.sub.3 phase and, after the B.sub.2 O.sub.3 phase has been eluted with a mineral acid, remains present in fine pores in the form of gel, which has to be removed by washing with alkali. However, it then is likely that SiO.sub.2 is also dissolved out, so that it is infeasible to effect a sufficient washing with alkali and obtain porous glass having a large volume of fine pores.
It is also known to produce heat and alkali durable porous glass mainly comprising an oxide of a rare earth element with use of a material glass of B.sub.2 O.sub.3 -Na.sub.2 O system glass-forming ingredients to which is added an oxide of a rare earth element and by operating a phase separation of the material glass, followed by eluting the B.sub.2 O.sub.3 phase with hot water [e.g., Journal of the American Chemical Society, 66, 221, (1983)].
However, this known process is not generally employable in that it requires using a particular material of an oxide of a rare earth element, and involves a disadvantage such that the size of pores in porous glass then obtainable is only so small as to be less than 200 angstrom.
Besides the above recited known processes, reportedly there have been attempts made to produce alkali durable porous glass by adding ZrO.sub.2 to a material glass, ZrO.sub.2 being usually blended as ingredient for imparting an alkali durability to glass. However, ZrO.sub.2 undergoes transfer into the soluble phase during the phase separation and becomes removed through a subsequently operated elution treatment with an acid, so that such attempts have not been successful.